Adsorption filters are used to remove undesired substances from gaseous or fluid mixtures. The present invention especially deals with filters for air filtration. Adsorbent particles, such as active carbon particles, clean undesired substances out of air carried through the filter. The adsorbent particles are often fixed or mounted on a supporting frame. Modern industrial developments have resulted in increasingly stringent requirements with regard to clean air. Such industrial developments are, for example, the manufacture of highly sensitive products, such as semiconductors and microelectronic devices. This type of manufacture is performed in clean-rooms with extremely high demands for particle cleanliness. In the mid nineties the increasing miniaturization reached a level where also gaseous chemicals resulted in yield damage. These yield-damaging chemicals are denoted as airborne molecular contamination (AMC). Semiconductor Equipment and Materials International (SEMI) in the standard SEMI F21-951 have given a classification system for AMC gases. This standard divides AMC into four classes: Acids, Bases, Condensables and Dopants. Ion exchange adsorbents can be used to filter all types of acidic or alkaline substances.
The earlier conventional adsorption filters for removal of an acid compound, such as HCl or a basic compound, such as ammonia have relied on a chemical impregnation that is loosely bonded to its substrate. The impregnation may under some circumstances even result in a contamination itself. These chemically impregnated filters have in some cases much lower capacity than an ion exchange adsorbent filter.
Another type of adsorption filter incorporating ion exchange particles has recently been developed, which uses ion exchange particles. Such an ion exchange particles containing adsorption filter is described in U.S. Pat. No. 6,402,819 B1. This filter comprises ion exchange beads fixed to a support made of reticulated polymer foam or a textile web.
In an adsorption filter using reticulated polymer foam as support, the adsorbent beads are fixed to the pore structure of the reticulated polymer foam. The pore structure of reticulated polymer foams has, due to its manufacture, a varying pore size, which leads to a varying adsorbent load. It is thus difficult to distribute the ion exchange beads evenly in the foam support, both in the surface direction and in the thickness direction. The adsorbent load also varies between different individual filters. Due to the uncertainty of the absorbent load of the filters, only a minimum adsorbent load and thus a minimum filtration capacity can be guaranteed to the user. This is a significant problem, since a portion of the adsorbent, which is often very expensive, may be unused. The open structure of a foam support is indeed beneficial from a pressure drop standpoint, but it makes it impossible to achieve high initial removal efficiency at the short contact times between air and filter structure necessary to minimize pressure loss over the filter. Furthermore, since the adsorbent load of the foam support depends on the pore structure, it is not possible to fine-tune the adsorbent load versus efficiency versus pressure drop characteristics.
Another type of support for the ion exchange beads that is also suggested in U.S. Pat. No. 6,402,819 is a textile web. A textile web support has the advantage that it can be pleated, which is advantageous with respect to the pressure drop. However, problems occur when such textile web structures are to be pleated to a filter element and mounted and fixed in a filter panel frame, since the filter element will become distorted when used in the filtering application.
There is thus a need for an improved filter structure that does not suffer from the above drawbacks.